Plasmonic metasurfaces supporting collective lattice resonances have attracted increasing interest due to their exciting properties of strong spatial coherence and enhanced light−matter interaction. Although the focusing of light by high-numerical-aperture (NA) objectives provides an essential way to boost the field intensities, it remains challenging to excite high-quality resonances by using high-NA objectives due to strong angular dispersion. Here, we address this challenge by employing the physics of bound states in the continuum (BICs). We design a novel anisotropic plasmonic metasurface combining a two-dimensional lattice of high-aspect-ratio pillars with a one-dimensional plasmonic grating, fabricated by a two-photon polymerization technique and gold sputtering. We demonstrate experimentally multiple resonances with absorption amplitudes exceeding 80% at mid-IR using an NA = 0.4 reflective objective. This is enabled by the weak angular dispersion of quasi-BIC resonances in such hybrid plasmonic metasurfaces. Our results suggest novel strategies for designing photonic devices that manipulate focused light with a strong field concentration.

This paper presents a comprehensive review of the literature and trends on the design of periodic microstructural composite materials including photonic crystals (PtCs), phononic crystals (PnCs) and metamaterials (MMs) by topology optimization. The properties of these materials rely highly on the constitutive materials and their spatial distributions and the resulting materials may exhibit various special properties, e.g., photonic/phononic band gaps, negative permittivity/permeability, negative effective modulus and negative refraction. Therefore, PtCs, PnCs and MMs can be viewed as structural materials with periodic unit cells and the design of which is a typical topology optimization problem for desired properties or functionalities. In recent years, a great amount of research has implemented topology optimization for designing these structural materials as well as the associated functional devices. This review summarises the most recent development in topology optimization of PtCs, PnCs and MMs and the possible directions for future research are recommended.

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In this work, a helical structure with three spiral channels is employed to build the coding units of metalenses, which can provide high transmission efficiency with an arbitrary phase shift compared to air. The helical unit with the phase shift of π is used as logical unit 1, and the hollow tube filled with air is regarded as logical unit 0. By arranging these logical units in specific sequences, acoustic metalenses can achieve wave-field manipulation like acoustic focusing and splitting. The focusing efficiency as high as 41.5% is achieved. Meanwhile, the genetic algorithm is applied to seek the optimal arrangement of the bipartite units for 3D sound focusing. Simulations and experiments are conducted to demonstrate the proposed coding metalenses for molding the acoustic wave field in the desired manners.